The combined history of genomes provides a glimpse at past evolutionary events, revealing selective forces that acted at all levels of cellular and organismal function. Although the individual gene and its immediate regulatory elements form the primary unit of selection, evolution does not stop there (1). Instead, selection can also act on entire groups of genes, leading to joint transfers of genes between genomes (2, 3), concerted gene loss (4), gene fusion events (5), coregulation of genes through common regulatory elements (6), and the creation and maintenance of operons containing nonhomologous but cotranscribed genes (7, 8).